1. Field of the Invention
The present invention relates to a steering system for a motor vehicle, and more particularly to a steering gear ratio changing system.
2. Description of the Prior Art
There has already been developed and laid open in the official gazette of Japanese Patent Application Laid-open No. 209362/1985 a steering gear ratio changing system comprising an input sun gear and an output sun gear whose numbers of teeth are unequal and which are respectively connected to an input shaft and a pinion shaft, wherein planetary gears are respectively meshed with the sun gears and are rotatably supported on a common gear carrier in a coaxial and unitary state, and wherein the gear carrier is rotated by a motor through a worm gear having a self-locking function.
In general, in a passenger motor car, ordinary steering torques lie in a range smaller than 1 kgfm, whereas a steering torque of or above 10 kgfm arises, for example, when a wheel tire has hit against a curbstone stone in a shocking manner.
Accordingly, in the gear ratio changing system as mentioned above wherein a steering torque loaded on a steering system is transmitted through the planetary gear device which constructs transmission ratio changing gearing, structural strengths need to be set on the basic condition that a transmission torque required of the transmission ratio changing gearing is larger than a transmission torque which can arise in the steering system, for example, the transmission torque ( about 10 kgfm) which arises at the shock of the wheel tire against the curbstone. This poses the problem that the planetary gear device cannot avoid being made unnecessarily large in size.